In order to prevent excessive rolling (which has an impact on vehicle attitude and handling) of a motor vehicle, especially during cornering, it is known to provide a torsion bar between the front wheels of a motor vehicle, and, in some cases, a second torsion bar between the rear wheels. However, during straight line motion of a vehicle and when the vehicle is off-road, the torsion bar can have a detrimental effect on comfort and wheel articulation. Semi-active roll control systems have been proposed which monitor various vehicle conditions. Such roll control systems include a locking device associated with the torsion bar and the wheels. When the sensed vehicle conditions indicate roll control is not required, the locking device is unlatched to effectively disconnect the effect of the torsion bar between the wheels. When the sensed vehicle conditions indicate that roll control is required, the locking device is latched to connect the wheels by way of the torsion bar. The latch position of the locking device determines the overall effect of the torsion bar on the wheels, and hence determines the level of roll control on the vehicle. In certain circumstances, it is possible to latch the torsion bar in the wrong position.